Fiber optic resonant ring sensor and source

ABSTRACT

Light from a laser diode is coupled into a fiber ring to narrow the line width. In one embodiment, Rayleigh backscattered light from the fiber is returned to the laser diode, and has the effect of greatly narrowing the diode output. In another embodiment the fiber ring is a double core ring, or a ring of plural spatially contiguous fibers. The laser diode is coupled into one core to narrow its line width, while the line-narrowed output is applied to another core which serves as a sensing device such as a gyro. Minimal or no compensation is needed to track the laser frequency to the ring resonance. In one embodiment, the first core is doped to produce a gain medium which lases at a center frequency that tracks the resonance of the sensing core as it drifts due to environmental effects.

BACKGROUND OF THE INVENTION

The present invention relates to fiber optic ring sensing devices suchas gyroscopes. This class of device operates by propagating a coherent,preferably polarized, light beam in an optical fiber which is coiled ina ring, .and sensing the shift in phase of the light traveling in thering. The ring may provide a multi-pass path with an effective pathlength tens or hundreds of times longer than the fiber length, resultingin high sensitivity.

In a basic device of this sort, the phase shift of light in the ring isdetected as the frequency difference between two beams travelling inopposite directions around the ring, e.g., via the beat frequency of atravelling beam and a commonly derived reference beam. Various beammodulating and signal processing techniques may be used to quantify thefrequency shift. In any case, the ability to resolve this shift in aninertial sensing instrument is dependent on the initial use of a purespectral source having a narrow line width.

The requirement for a narrow line width has generally required the useof gas or crystal laser sources, and has ruled out the use of laserdiodes as sources for a fiber optic ring sensor instrument.

However, because laser diodes are extremely inexpensive, the developmentof a narrow line width laser diode system for a fiber optic sensorremains a desirable goal.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a fiber opticring is coupled to the lasing cavity of a laser diode to narrow its linewidth. The laser diode is directly coupled to a fiber optic ring and thediode line width is narrowed by feedback of light traveling in the ring.The ring may be incorporated in the sensing ring of an instrument, thusproviding a new and economical sensor architecture. In one aspect of theinvention, a novel laser source is provided wherein one side of thelaser diode cavity is coupled without an isolator into a fiber opticring, and is line-narrowed by the feedback of Rayleigh backscatteredlight from the ring. The fiber ring may be the sensing ring of aresonant fiber optic gyro (RFOG).

In another aspect of the invention, the laser diode may be coupled intoa fiber core which is physically contiguous in a single coil, with theoptical fiber sensing path of a FORG. This may be accomplished byemploying a coil made from a double core fiber, wherein a first core ofthe fiber provides the line width narrowing path, and the line-narrowedlaser output of the laser diode is coupled into the second core of thefiber to perform the sensing operations of the instrument. An alternateconstruction of this embodiment is achieved by winding two separatefibers together in a coil such that both fibers are contiguous andtherefore subject to the same physical e.g., thermal and acousticenvironment. In various constructions according to this aspect of theinvention, the line narrowing fiber may be set up to provide Rayleighbackscattered light at a single side of the diode cavity, or may operateas a direct traveling wave feedback path between opposed ends of thecavity. In another embodiment of this latter type, the fiber may bedoped to achieve a lasing effect, and the diode may serve simply to pumpthe lasing fiber.

By using a fiber ring for laser source line narrowing in this manner,the thermal controls to effect tracking corrections are essentiallyreduced to a common set of circuitry for the source and rotation sensingring, without the need to both perform ring measurements and tune thelaser to compensate for the different thermal characteristics of theseparate laser and external cavities employed in prior art laser drivenRFOGS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B show two prior art laser diode narrow line sources;

FIG. 2 shows a laser diode source stabilized in accordance with thepresent invention;

FIG. 3 shows a preferred embodiment of a ring sensor utilizing the lasersource of FIG. 2;

FIG. 4 shows a ring sensor architecture according to another aspect ofthe invention;

FIGS. 5 and 6 show different embodiments of a ring-stabilized laser.diode sensing instrument; and

FIG. 7 illustrates the mechanical layout for a device such as shown inFIGS. 4-6.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B illustrate known prior art approaches to construction ofline-narrowed laser diode light sources. In FIG. 1A a Fabry-Perotexternal resonant cavity 2 is coupled to a laser diode 1 to act as anarrow band filter that returns only a selected wavelength to the lasingcavity of the diode. The external cavity may take the form of a fiberhaving one silvered end 3, and a partially reflective end 4 coupled tothe laser diode. The feedback wave varies as the dimension of theexternal cavity, i.e., the fiber length, changes, and varying degrees ofline narrowing are achieved by varying the fiber length and couplingparameters.

In the construction illustrated in FIG. 1B, the ends of the lasingcavity of a diode 1 are partially transmitting, and a fiber ring 6interconnects the two ends with each other to provide a traveling wavefeedback. The fiber plus the laser cavity together thus constitute aresonant ring, with all the gain lumped in the diode cavity. An outputinto a fiber 7 is obtained via a coupler 16 along the ring. In thisconstruction the principal mode varies in accordance with the sum ofdimensional changes of the diode cavity and the fiber length. For thisreason, precise control of the output frequency requires separatemonitoring and compensation circuitry to correct parameter variationsfor both the diode and the fiber.

In either of the constructions shown in FIG. 1A or 1B, it is theforward-propagated light in the fiber, either as reflected light fromthe far end 3 of the fiber (FIG. 1A) or as returned light passingthrough that end (FIG. 1B), which provides the feedback necessary for acoherent narrow line laser operation. Further, in each case, stableoperation is dependent upon the resonant modes of the fiber cavity.Environmental effects on the fiber (e.g. temperature, acoustics, etc.)will cause instabilities in the laser source output frequency. Whenusing such a prior art system as a laser source for an instrument or acommunications line, an optical isolator is generally required at theoutput 7.

In contrast to these prior art constructions, applicant has found thatan entirely different approach produces a high degree of line narrowingwith significant simplifications of instrument design.

FIG. 2 shows a stabilized laser diode source 10 in accordance with oneaspect of the present invention, wherein a laser diode has a lasercavity 12 with one end wall 14 coupled to a multi-pass fiber ring 15 insuch a manner that only backscattered light from the fiber returns tothe diode cavity 12. The ring 15 preferably has a reasonably highfinesse, e.g. F=100, so that a twenty-five or thirty-five meter fibercoil presents an effective path length of several kilometers. A coupler16 couples the light from the diode cavity 12 into the ring 15, with acoupling ratio that preferably couples a high proportion of theforward-propagating light into the fiber, and allows return of a highproportion of the backscattered light to the diode cavity. Indistinction from the prior art configurations discussed above, noforward-propagating light is returned to the diode, so that theline-narrowing feedback consists essentially entirely of Rayleighbackscattered light. Applicant has found the line-narrowing effect ofbackscattering from a tightly coupled very long fiber path in thismanner to result in very narrow diode line width.

In one preferred embodiment, the fiber ring used as an external lightpath to narrow the diode line width also functions as the ring resonatorof a fiber optic ring gyro. The forward-propagating light is processedto determine inertial data, while the Rayleigh backscattered lightreturns to the lasing cavity of the diode as described above to narrowthe line width of the diode output.

FIG. 3 shows such an embodiment of a fiber optic ring gyro system 60 inwhich the sensing and feedback rings are identical. In this embodiment,a laser diode source 20 has an output path 21 which is coupled withoutany optical isolator into a resonant ring 40 used as an inertial sensor.By way of completeness, an entire system is shown, which, in alternatetime intervals, directs clockwise and counterclockwise beams into thering 40, with conditioning elements 30 and detection elements 50. Asshown, the laser diode source 20 comprises a laser diode 1 with itsoutput along path 21 coupled via coupler 16 to an output sensing andcontrol circuit 22 which adjusts the diode current provided along line23 to maintain a stable diode output. Control circuitry 22 may receivecontrol signals 24 from ring signal processing circuitry, and otherfeedback in a known manner. Output path 21 couples via fiber opticconnector 26 into a path along switching, condition and detectionelements that is bidirectionally coupled via a resonant coupler 41 intoresonant ring 40. The connector 26, and other like connectors 26a, 26band 26c serve only to indicate the generally modular or separate natureof the different sub-assemblies or components of a FORG. For expositorypurposes, the various successive path segments appearing between theconnectors 26, 26a, 26b, 26c may, except in the discussion of switchingbelow, be regarded as one continuous optical fiber and the connectorsthemselves disregarded.

Returning to FIG. 3, the laser diode output on path 21a passes throughdiscriminant generation element 31 and proceeds along path 21a to anelectro optic switch 32 which operates in time division multiplexdefined by a control signal on line 33 to direct the light either into afirst path 21b or a second path 21c. Viewed in the input sense, thesewill be referred to as the CCW input path and the CW input path, since,as is apparent from the drawing, the forward travelling light in thesepaths couples via coupler 41 into ring 40 in counterclockwise andclockwise senses, respectively. The various signal conditioning anddetecting elements positioned along the CCW or CW lines are identical,and only one set will therefore be described, it being understood thatin alternate cycles alternate ones of them are used to perform the samegeneral functions on one of the two (CW or CCW) beams.

Light traveling along path segments 21b, 21b' is frequency tuned with asawtooth phase modulation waveform by serrodyne driver 34a and issuppressed carrier phase modulated by modulator 35a before passing toresonant coupler 41 and coupling into ring 40 as a counterclockwisepropagating beam, denoted by the long arrow CCW. The CCW beam travelingin the ring is coupled out by the same coupler 41 and proceeds to theleft as viewed in the drawing along the lower path segment 21c'. A 50/50output coupler 16a diverts a portion of this light to the CCWdetector/preamplifier 36a, the output of which is fed along line 37a toa signal processor as shown. The remaining portion of the CCW beamproceeds unused along path segments 21c' 21c and 21d. While the CCWinput fiber 21b is being used, Rayleigh backscattered light from thering 40 propagates in a clockwise direction around the ring, asindicated by the small arrow labelled RBS, and is coupled back out viaresonant coupler 41 to travel in the reverse direction back along path21b', 21b, 21a and 21 to the laser diode. In this manner, the FORGsensing ring itself provides feedback to narrow the line width of laserdiode 1.

In alternate cycles, light along the input path 21 is coupled intosegments 21c, 21c' to provide a CW beam to the ring. In that case, beammodulating elements 34b, 35b, and via coupler 16b the sensing element36b are operative, and a stabilizing CCW-backscattered signal proceedsback along path 21c', 21c, 21a and 21 to the laser diode. Thus in bothCW and CCW operation, the laser diode is stabilized by Rayleighbackscattered light directly from ring 40.

The serrodyne signal frequency applied to 34b and 35b is fixed, whereasthe frequency applied to 34a and 35a is modulated by the signalprocessing to track the CCW resonant frequency of the fiber ring. Itwill be observed that in the architecture of the device of FIG. 3, allelements to the right of laser input connection 26 are conventional.Significantly, however, a simple laser diode and its current controlfeedback loop for tracking the CW resonant frequency are the only inputto the device, and unlike most RFOG designs, an optical isolator isneither required nor desirable. A hybrid design is also feasible, usingthe backscatter-stabilized source of FIG. 2 in place of the diode source20 of FIG. 3. In that case, one or more isolators may be used, and thesensing ring 40 may operate in an entirely conventional manner.

A second and related embodiment, employs a fiber ring which is eithermade from a double-core fiber, or consists of a pair of fibers woundtogether to form a coil in which the two fibers are contiguous andfollow substantially identical winding paths so that they are subject toidentical environmental (e.g., thermal and acoustic) conditions. In thisembodiment, one core or fiber is coupled to a laser diode as aline-narrowing external resonant cavity, which causes the laser diodeoutput to have a narrow line spectrum. This output is coupled into thesecond fiber or core of the ring and is subjected to signal processingof a known type, such as a serrodyne modulation, mixing, anddemodulation, to detect a frequency shift indicative of ring rotation.In this embodiment, the feedback ring preferably returns a direct,forward propagating feedback signal to the diode cavity, rather than aRayleigh backscattered light signal. However, great simplifications incontrol and compensation circuitry are achieved since the mode shifts ofthe first core are identical to those induced in the second core bythermal, acoustic or other environmental effects. Thus one set offrequency control circuitry may compensate both the sensing fiber andthe feedback fiber.

The advantages of this construction may be understood as follows. Theline width Δν of a laser tuned by a resonant cavity is related to thepassive resonator full width half maximum, Δν_(1/2), according to theequation ##EQU1## Where P is the optical power generated in the lasingmedium, N₂ (N₁) is the population density of the upper (lower) laserlevel, and hν₀ is the energy per photon. The quantity Δ.sub.ν1/2 isgiven by: ##EQU2## where 1 and α are the cavity length and loss perpass, and c is the velocity of the light in the medium. Equations (1)and (2) apply equally well for linear external cavity diode lasers andring resonator narrowed diode lasers. A high performance gyro willrequire that Δν be minimized to perhaps below 20 kHz.

From Equation (1) it follows that narrowest laser line widths areobtained from a resonator with a narrow line width. Equation (2) impliesthat, when cavity losses are largely independent of length (such as whencoupler losses dominant over fiber rings losses), narrower line widthscan be obtained for longer cavity lengths. The ring resonator can bemade 35 meters or more in length, and thus will produce narrower diodelaser line widths than are available from other external cavityconfigurations. For these reasons, a ring resonator narrowed diode laseroffers advantageous packaging and performance capability for a highperformance gyro.

Turning now to the second aspect of the invention, FIG. 4 illustrates inbare schema the architecture of a FORG wherein two different butessentially unseparated ring paths 81a, 81b are used for line-narrowingand for inertial sensing, respectively. A laser 100 is coupled to afirst ring 81a by a coupling arrangement indicated schematically by 102to achieve a narrow line width, and the laser output is coupled viaoutput coupler 85 and fiber C into the second ring 81b which serves asthe sensing ring of an RFOG. The coupling arrangement may take the laseroutput directly from ring 81a, and may include source monitoring andconditioning elements of the type described above. Similarly, lightwhich has traversed ring 81b is detected and demodulated by RFOG outputprocessing elements which, as in the preceeding Figure, may overlap tosome extent with the monitoring and conditioning elements along theinput coupling path.

As illustrated, the clockwise propagating beam in ring 81a is coupledout along a fiber C to a signal conditioning module 82 which frequencyshifts and modulates the light from input fiber C to provide twodistinct signals on fibers A and B which are coupled in clockwise andcounterclockwise senses, respectively, into ring 81b by coupler 84. Theclockwise and the counterclockwise light circulating in ring 81b areeach coupled out via the same coupler traveling in the opposite sense,into the other fiber, B and A, respectively, and coupled via couplers83b and 83a to photodetectors D₂ and D₁ for demodulation. This couplingarrangement 102 in this Figure is indicated only schematically, since,as discussed in greater detail below, different physically distinctcouplings may be employed to narrow the line width, with correspondingdifferent advantages ranging from improved packaging and performance toreductions in processing or control circuitry.

FIG. 5 illustrates in greater detail one embodiment 120 of a dual ringRFOG having the general architecture illustrated in FIG. 4. In thisembodiment a first ring 121a is used in a direct path between the endsof a laser diode cavity 12, thus providing a narrow line source of thetype illustrated in FIG. 1B, and light is coupled out of the ring 121aby a coupler 16d as the input source to an inertial sensing ring 121b.The various elements for modulating the light fed to ring 121b anddetecting and demodulating light that has traversed the ring arenumbered identically to the corresponding elements shown in FIG. 3, andtheir operation requires no further explanation. However, certain ofthese elements may be fabricated as portions of an integrated opticschip 39, and this grouping is indicated in the Figure, and will bediscussed further below.

In this embodiment rings 121a and 121b are separate cores of a singlefiber, or are contiguous fibers which have been wound in a single ringso that their cores are separated by no more than a few hundred micronsat corresponding points along their lengths. Accordingly, when thermalexpansion of ring 121b causes a lengthening of the sensor's resonantpath, requiring a change in the laser input frequency to maintain astable mode, a substantially identical change in path length of ring121a occurs. As a result, the resonance of the path formed by ring 121awill tune the laser by approximately the required amount to maintain astable mode in ring 121b.

In one further embodiment of a laser diode RFOG using this double patharchitecture, the ring has a core which is itself a lasing medium. Thecore may, for example, be an Erbium-doped silica core which emits lightat 1.5 microns. As in the embodiment of FIG. 6, both cores of the ringare essentially contiguous. In this further embodiment, illustrated inFIG. 6, the laser diode 1 serves only as a pump laser for the fiber ring129a, and the output of the ring 129a is used as the input to sensingring 129b. This construction substantially eliminates the need tocompensate for the small drift that would occur in the source of FIG. 5caused by the different thermal properties of the laser diode gainmedium compared to the fiber thermal properties. In fact most problemsassociated with coupled resonator cavities, such as feedback andfrequency tuning, are removed. Furthermore, much complexity of RFOGtuning and tracking is eliminated, since the laser center frequency ofthe pumped fiber core 129a tracks the resonance of fiber core 129b astheir dimensions change. In fabricating such a double-core or two-fiberring it may be desirable to alter the material used for the fiber 129bto assure that both light paths have substantially identical thermalexpansion and optical index characteristics.

Further advantages of the intimate double light path RFOG architecturewill be apparent from FIG. 7, showing the mechanical layout for the RFOG120 of FIG. 5. The two cores or fibers 121a, 121b of FIG. 5 which wereillustrated at separate ends of that Figure for clarity of exposition,are, as described above, virtually unseparated, except for such shortdivergent portions as are required for passing through a coupler orthrough an optical processing element required by the instrument. Asshown in FIG. 7, this provides a compact mechanical layout wherein bothcores/fibers 121a, 121b together with the laser diode 12, both couplers16d, 41 and couplers 16a, 16b occupy the perimeter of a disc. Thephotodetector/preamplifiers 36a, 36b, and an integrated optics chip 39which performs the path switching and beam modulation occupy the centralregion, so that the spatial requirements of the FORG are essentiallyreduced to the space required for one sensing coil. Thus, the dual coredevice of FIG. 4 has significant packaging, as well as circuit design,advantages.

This completes a description of the invention, the teachings of whichhave been illustrated by reference to different embodiments of theseveral aspects thereof. Provided with these teachings, variations andmodifications will occur to those skilled in the art, and all suchvariations and modifications are considered to be within the scope ofthe invention in which patent rights are claimed, as defined by theclaims appended hereto.

What is claimed is:
 1. In a resonant fiber optic gyro of the typewherein laser beams derived from a laser source are launched into afiber ring to develop signals indicative of a frequency shift induced bya component of rotation of the inertial frame of the ring, and includingfeedback means operative on a said beam to maintain its frequency in aresonant mode of the fiber, the improvement wherein the laser sourceincludes a laser diode which emits said beam as an output beam, andmeans for coupling said beam into the fiber ring such that only Rayleighbackscattered light from the ring returns to the laser diode and narrowsthe line width thereof.
 2. The improvement of claim 1, wherein the fiberoptic gyro contains a fiber ring having first and second parallel lightconducting fiber cores, and said laser diode output beam is launchedinto and stabilized by light returned to the diode from said first core,and the forward propagating light of said first core is provided as aline-narrowed input beam to said second core.
 3. A laser sourcecomprising a laser diode having a lasing cavity with one end thereoftightly coupled into a resonant fiber optic ring such that the diodereceives only Rayleigh backscattered light from the ring, in an amounteffective to narrow the line width of light produced by the laser diode,forward propagating light in the ring constituting a line narrowedoutput of said laser source.
 4. The source of claim 3, attached as aninput to a fiber optic sensing device.
 5. An improved fiber opticsensor, of the type wherein laser beams derived from a laser source arepropagated in a fiber ring to develop signals indicative of a physicalstate sensed by the ring, and including feedback means operative on asaid beam to maintain its resonant frequency in a resonant mode of thering, wherein the improvement resides in that the fiber ring is aresonant ring formed as a coil having first and second essentiallycontiguous light conducting cores and said first core is coupled to alaser diode cavity to narrow the line width of the laser diode, the linewidth narrowed output of the laser diode being provided as an input tosaid second core, said laser beams being propagated in said second coreto develop said signals.
 6. An improved fiber optic sensor according toclaim 5, wherein said first and second cores are cores of a singlefiber.
 7. An improved fiber optic sensor according to claim 5, whereinsaid first and second cores are cores of respective first and secondfibers which are contiguously wound together in a ring along a singlewinding path.
 8. An improved fiber optic sensor according to claim 5,wherein said first and second cores are single mode, polarizing, orpolarization maintaining cores.
 9. An improved fiber optic sensoraccording to claim 5, wherein said first core is coupled between opposedends of the laser diode cavity to provide a coupled resonator fornarrowing the line width of the diode.
 10. An improved fiber opticsensor according to claim 5, wherein said first core is formed of alasing medium which is pumped by the laser diode and lases at afrequency different from that of the laser diode.
 11. An improved fiberoptic sensor according to claim 10, wherein said second core has thermalcharacteristics matching said first core, so that the ring laserfrequency tracks the resonance of the second core.
 12. An improved fiberoptic sensor according to claim 5, wherein said first core is coupled toprovide only backscattered light to said laser diode for narrowing saidline width.
 13. An improved sensor according to claim 5, wherein saidsensor is a resonant fiber optic gyro.
 14. An improved method ofproviding a coherent source of light, wherein the improvement comprisesthe steps ofi) coupling a light output of a laser diode as a forwardpropagating beam along a path into a resonant fiber optic ring such thatthe laser diode receives only Rayleigh backscattered light along thepath in an amount effective to narrow its line width, and ii) coupling aportion of said forward propagating beam as a light output thusconstituting a line narrowed coherent source of light.
 15. An improvedmethod of performing high resolution sensing of an inertial parametervia a resonant fiber optic ring, wherein the improvement comprisesforming said ring with first and second fiber cores in parallel along anessentially common contiguous winding path, circulating light in saidfirst core to develop a reference laser source, and providing saidreference laser source to said second core for sensing of the inertialparameter.
 16. The improved method of claim 15, wherein the step ofcirculating light in said first core includes coupling said first coreas an external Fabry-Perot cavity to a laser diode to narrow the laserdiode line width.
 17. An improved method of sensing a parameter with aresonant fiber optic ring, wherein the improvement comprises coupling alaser diode to the ring to receive only Rayleigh backscattered lighttherefrom in an amount effective to narrow the line width of the laserdiode thereby achieving a narrow line output, and applying said narrowline output to develop a sensing beam.